Avermectin and milbemycin compounds are known for having superior anthelmintic and antiparasitic effects. For example, see U.S. Pat. Nos. 4,310,519 to Albers-Schonberg et al., 4,378,353 to Goegelman et al. and 4,199,569 to Chabala et al. which disclosed avermectin compounds and 3,950,360 to Aoki et al. which disclosed milbemycin compounds. The use of said compounds is growing parasitised animals generally results in a weight increase being due to the general improvement of the health of said animals as the parasite burden is reduced. These known compounds have been found to possess a novel utility by reducing the transmission of certain infectious disease organisms prior to and after birth and increasing litter size in monogastric domestic animals after treatment about mid-gestation of gravid female with said compounds.